A nut and bolt combination is a well known fastening device. Generally speaking, a metal washer encircles the bolt shaft and is interposed between the nut and the member against which the nut is tightened. Location of a washer between rotatable and static members is known to promote a fit of the fastened members more resistant to loosening by vibration. The nuts, bolts and washers in common use are often manufactured of metal but modification of this arrangement has been suggested for various purposes over the years.
For example, U.S. Pat. No. 2,982,573 of McKee, Jr. describes a composite sealing washer having an annular sealing portion of neoprene rubber received within a cup-shaped flexible metal member. A washer is used in conjunction with a conventional nut and bolt to fasten two plates together. The sealing portion is oversized such that when the nut is tightened onto the bolt, material of the sealing portion spreads into a gap between the bolt and plates.
U.S. Pat. No. 1,208,620, of Moser, discloses a washer having a gasket received within a countersink in one of its sides. The gasket is said, among other things, to assist in giving a better binding action of the washer against the nut. It is suggested that two such washers be used in conjunction with an ordinary nut and bolt to clamp together two elements. The gaskets are arranged to abut surfaces of the elements. The gasket is of rubber or other resilient material and is oversized so as to be compressed when the arrangement is tightened by rotation of the nut along the bolt.
One longstanding problem with the nut and bolt fastening arrangement sometimes arises when a nut is fastened too tightly onto the bolt. Overtightening can lead to stripping of the threads of one or the other or both of the nut and bolt. Sometimes, the shaft of the bolt is snapped off. Although nuts and bolts are generally considered low-cost items in themselves, it can be very time-consuming to have to replace an awkwardly positioned bolt, or one that is part of a complex arrangement of parts that must be disassembled in order to replace the bolt.
If relatively soft materials are fastened together by squeezing the materials between the nut and bolt, overtightening can sometimes lead to the head of the bolt being "pulled through" the material and to damage of the material.
Threaded fasteners such as nuts and bolts are sometimes used in arrangements in which layered members of material are sandwiched together between the nuts and bolts. Such an arrangement is shown, for example, in U.S. Pat. No. 5,237,789, issued Aug. 24, 1993 and naming the same inventor as this patent application. The previous patent specification, which is incorporated herein by reference, shows a clamp for a roof device in which a roofing membrane is sandwiched between an underlying skirt of a roof device (such as a pipe sleeve) and an overlying clamping ring. The ring, membrane and skirt are sandwiched between the nut and head of a bolt having its shaft passing through registered apertures of the various elements, the fastening nut being rotated down against the upper surface of the clamping ring. An object of such an arrangement is to provide a seal against the passage of water between the membrane and skirt without the use of adhesive.
Such roofing membranes are commonly of sheet materials such as PVC (polyvinyl chloride), EPDM (ethylene propylene diene monomer polymers), Hypolon, neoprene, etc. It has recently been found for certain of these roofing membranes, particularly those lacking "memory", such as PVC, that it is beneficial to interpose an annular sealing gasket between the clamping ring and membrane in order to obtain an improved seal against water ingress.